Polyoxyalkylene glycol derivatives have recently been reported as being effective as a lubricating oil for a refrigerator employing a hydrogenated Flon refrigerant such as Flon 134a without a fear of causing environmental pollution problem by virtue of their high miscibility with the above-mentioned hydrogenated Flon refrigerant (Specification of U.S. Pat. No. 4,755,316).
Nevertheless, the polyoxyalkylene glycol derivatives are poor in wear resistance and increase the wear between aluminum material and steel material in a refrigerator in an atmosphere of aforementioned refrigerant, thus causing serious problem in practical application. The frictional surfaces between aluminum material and steel material include the contact surface of a piston and piston shoe for a reciprocating compressor, and that of a vane and housing for a rotary compressor. In either case, lubrication is important for the friction surfaces.
On the other hand, the conventional extreme pressure agents which have heretofore been used in refrigerator oil impaired the stability of the oil and suffered insufficient wear-resistance effect, making themselves impossible to endure practical application.